In connection with injection moulding, compression moulding and extrusion of plastic components, the tool is exposed to corrosive media originating from the components of the plastic, but also from the release and lubricating agents that are applied onto the tool surface in order to decrease the friction between the plastic and the forming tool. Cooling ducts with water and its normal content of chloride ions are known to result in corrosion damages in forming tools for plastic. Often, the tools have a complex shape with cavities. Even when a tool is taken out of operation, the liquid remaining in these cavities can result in local attacks of corrosion if the material does not have the requisite corrosion-resistance. Galling and fretting are other fields of problems that result in increased maintenance and decreased production.
Galling and adhesive wear is caused by micro-welding between tool parts when exposed to a high contact pressure that leads to metal fragments getting stuck on the tool parts and thus increasing friction. Eventually, shearing occurs between the parts, which results in complete renovation or exchange of these.
Fretting or fretting corrosion takes place between parts that are exposed to vibrations or cyclic movements in connection with the forming cycle. Discoloration of the form parts due to corrosion products will result in impaired functionality and also to discoloration of the plastic products. In order to avoid these problems the tool parts must be polished, which means that in time they will lose tolerance and new tool parts must be acquired.
A known tool material that is manufactured by the applicant and that is used in the present technical field is the melt metallurgically manufactured forming steel for plastics that is known under the trade name Stavax ESR®, having the nominal composition 0.38 C, 1.0 Si, 0.4 Mn, 13.6 Cr, 0.30 V, 0.02 N, balance iron and normal impurities. This steel has a good corrosion resistance and a very good finishing quality.
Yet another known tool material that is manufactured by the applicant and that is used in the present technical field is the melt metallurgically manufactured forming steel for plastics that is known under the trade name Stavax Supreme®, having the nominal composition 0.25 C, 0.35 Si, 0.55 Mn, 13.3 Cr, 0.35 Mo, 0.35 V, 0.12 N, balance iron and normal impurities. This steel has a carbide content of about 0.5% by volume and has a very good corrosion resistance and a very good finishing quality.
Another known tool material that is manufactured by the applicant and that is used in the present technical field is the melt metallurgically manufactured forming steel for plastics that is known under the trade name ELMAX®, having the nominal composition 1.7 C, 0.8 Si, 0.3 Mn, 18.0 Cr, 1.0 Mo, 3.0 V, balance iron and normal impurities. This steel has a good corrosion resistance and the wear resistance is good too, but it is desirable to further improve the properties. Depending on heat treatment, the steel normally has a highest hardness of 57-59 HRC in the hardened and tempered condition, which under certain conditions may be too low, resulting in impression damages when the tool is used, e.g. due to fragments of plastic that may be released when opening the tool and ending up between the tool halves when these are pressed against each other in the next forming operation.
Cold-working often comprises cutting, punching, deep drawing and other types of forming of metallic work pieces, usually in the form of sheets and normally at room temperature. Cold-working tools are used for this type of operations, on which tools a number of demands are put, which are difficult to combine. The tool material should have a good resistance against abrasive wear, an adequate hardness, and for some applications it should also have a good resistance against adhesive wear and also an adequate toughness in its working condition.
Sverker 21® is a conventionally manufactured steel with the composition 1.55 C, 0.3 Si, 0.3 Mn, 11.8 Cr, 0.8 Mo, 0.8 V, balance iron and impurities at normal contents, which steel has been widely used for cold-working and other applications.
The above mentioned steel, and other steels on the market, fulfil high demands on abrasive wear resistance and toughness. They do however not fulfil very high demands on adhesive wear resistance, which is often a dominating problem in different types of cold-forming tool applications, such as sheet pressing, pipe bending and cold forging of e.g. martensitic or ferritic steels, sheets of austenitic and ferritic stainless steels, copper, brass, aluminium etc. Such problems can be decreased by lubricating and/or coating, for example by PVD or CVD techniques, of the tool surfaces by friction-lowering ceramic layers of e.g. TiN, by surface nitration or by coating with hard chromium, but such solutions are expensive and time-consuming. Moreover, there is a major risk of damages on and/or flaking of the layers. Reparation becomes very complicated if abrasive or adhesive wear damages occur, as the damage is always on a part of the tool having a high strain. Abrasive and adhesive wear also occurs between different tool components.
In addition to the above mentioned properties, the tools should have very good corrosion resistance, high hardness, good wear resistance, good grindability, good machinability and high finishing quality, good dimensional stability, high compression strength, good ductility, good fatigue strength properties and high purity.
By solid phase nitration, powder metallurgically made materials can be given a high content of nitrogen, whereby they achieve a built-in nitrided layer. One example of such a material is the applicant's own steel that is marketed under the name VANCRON 40®, which is comprised inter alia in Swedish patent no. SE 514,410, having the following ranges of composition, in % by weight, 1-2.5 C, 1-3.5 N, 0.05-1.7 Mn, 0.05-1.2 Si, 3-6 Cr, 2-5 Mo, 0.5-5 W, 6.2-17 (V+2Nb), balance iron and unavoidable impurities at normal contents.
It is known from the article “Influence of nitrogen alloying on galling properties of PM tool steels”, 6th International Tooling Conference, Karlstad Universitet 2002, that nitrogen, by together with carbon combining with vanadium in order to form M(C, N) carbonitrides and M6C carbides, has a positive effect on the anti galling properties of a tool steel.